The quantum mechanical phenomenon of “resonant tunneling” was first analyzed in 1969 by Esaki and Tsu in 1969 (Esaki et al., IBM J. Res. and Develop. 14:61–69 (1970). The concept of “resonant tunneling” has since evolved into that of the “resonant tunneling diode” (RTD), wherein a central region containing some central moiety, for example a quantum dot, is placed between two quantum mechanical tunneling barriers. Two conducting electrodes in contact with the two quantum mechanical tunneling barriers can therefore allow the injection of electrical current from a first electrode, across a first barrier to the moiety, and from the moiety across a second barrier to the second electrode.
If an energy level in the central moiety matches the electron energy in the first electrode, some enhancement of electrical current through the RTD occurs. This phenomenon can be called matched-level resonance.
If the matched-level resonance condition is present, and if in addition the two quantum mechanical tunneling barriers are equal in magnitude, a tremendous additional enhancement in electrical current through the RTD occurs. This second phenomenon wherein the two quantum mechanical tunneling barriers are equal in magnitude can be called matched-barrier resonance.
A variant of the resonant tunneling diode is used with a scanning tunneling microscope (STM) in a procedure called “resonant tunneling spectroscopy,” which has been refined into a procedure called “shell tunneling spectroscopy” (Bakkers, et al., Nano Letters, 1(10):551–556 (2001)).
The prior shell-tunneling spectroscopy work has been limited because the sample under test is fixed in place in a single position on top of an insulator of constant thickness. In order for the desirable phenomenon matched-barrier resonance to be employed in such a device, the magnitude of the upper quantum mechanical tunneling barrier due to the separation of the STM tip from the sample under test must be matched to the magnitude of the lower quantum mechanical tunneling barrier due to the presence of the insulator between the sample under test and the conducting substrate, and this is difficult to achieve in practice.
Thus there exists a need for a tunneling spectrometer in which the phenomenon of matched-barrier resonance can be effectively employed, and at the same time the phenomenon of matched-energy resonance can also be made to occur, in order to obtain a complete density of states for a test sample. The present invention addresses this need.
Relevant Literature
Bakkers et al., Nano Letters, 1(10):551–556 (2002); Chang et al., Applied Physics Lett., 24(12):593–595 (1974); Goodhue et al., Applied Physics Lett., 49(17):1086–1088 (1986); Sollner et al., Applied Physics Lett., 43(6):588–590 (1983); Esaki et al., IBM J. Res. Dev., 14:61–65 (1970); and Sollner et al., Applied Physics Lett., 45(12):1319–1321 (1984).